Weighing systems for producing a product charge of precise weight are well known in the art, and include systems that employ one of several combination weighing techniques. In general, these systems obtain a precise weight product charge by combining the contents of a plurality of buckets or hoppers, each of which contains a portion of the product charge. A controller reads the weight of each hopper, searches for the hopper weight combination which most closely approximates the desired weight, and subsequently dumps the contents of the corresponding hoppers.
Known combination weighing devices include the weighing apparatus disclosed in U.S. Pat. No. 4,515,231. That apparatus is characterized by a serial weighing operation in which articles are first provided to a plurality of fill-to-weight cutoff pool hoppers. After the weight of the product charges in the pool hoppers have been determined, the contents thereof are fed to corresponding weighing hoppers. The weight of product in each of the weighing hoppers is then measured and a computer determines that combination of hoppers whose weight sum is equal or closest to a desired product charge weight. The '231 apparatus is most useful in handling light products or products apt to cling together in bunches. However, that apparatus is burdened by two discrete weighing operations. Moreover, the speed of operation of the combination weigher is necessarily limited by the filling speed of the fill-to-weight cutoff pool hoppers.
U.S. Pat. No. 4,418,771 discloses a combination weighing method and apparatus wherein quantities of the product of targeted weight are distributed to a plurality of scales for accurate weighing. The weighed product charge is fed from each of the scales to a plurality of storage cups associated with each of the scales and the product weight associated with each storage cup is registered. Specific combinations of storage cups are tested to determine whether the combined product weights add to make the desired weight within acceptable limits. The method and apparatus is characterized by a controller which will examine only a subset of scales rather than all of them to find a combination which will provide the desired weight product charge. If no combination produces a total product charge within selected weight limits, the controller will broaden the limits and repeat the cycle.
Known combination weighing systems which will generate a product charge from two or more different articles include the weighing system disclosed in U.S. Pat. No. 4,473,126. The weighing system includes a separate conventional combination weigher for each of the different articles which comprise the product charge. U.S. Pat. No. 4,549,617 teaches a combination weighing system having a method of setting a target value (weight or number) for each of the component articles and supplying a plurality of combination weighing machines. For each category of articles and from all combinations of the weighing machines, a controller determines the best combination whose total value (weight or number) is equal to the target value set for that category.
The combination weighing method disclosed in U.S. Pat. No. 4,522,274 obtains a product charge having a mixture of articles of different categories wherein the articles in each category are contained in the mixture at a number ratio approximately equal to a target number. The method includes the steps of setting a number for each category to provide a number ratio closest to a target category number ratio, and measuring the total weight of a primary mixture obtained by extracting the set number of articles in each category and mixing these articles. If the weight of the primary mixture is initially outside of a minimum, a controller computes the weight difference between the primary mixture and target weight and provides supplemental articles to make up the short fall. The method provides for uniformity in the product weight-to-target number ratio as opposed to component distribution.
The combination weighing device taught in U.S. Pat. No. 4,388,975 simultaneously weighs a plurality of articles in weighing balances and selects a plurality of the weighing balances to comprise a total weight of articles within a predetermined range. The device is characterized by a controller which causes one or more of the specified weighing balances to always enter into the selected combination to insure that each package contains articles from selected weighing balances.
One of the problems associated with combination weighing systems is that there is a finite number of times that the scales will not present at least one combination that is within the acceptable weight range for a product charge. When this happens known combination weighers simply output an empty bag or cause a bagging cycle to be skipped entirely. Such an occurrence is known as a "miss".
For high volume application the number of misses can be substantial. For example, for metalized tobacco pouches where a weigher may package approximately twenty million packages per year, there will be approximately three hundred thousand throw away packages because of missed filling cycles.
It would be advantageous to have a combination weigher which provides for a mechanism that will supply a product charge during those times when the combination weigher is unable to come up with a product charge within the acceptable product charge quantity range. The present invention is drawn to such an apparatus.